30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

Current status of the expanded GRAPES collaboration experiment at Ooty in India.
S.K. GUPTA1 , H.M. A NTIA1 , A. I YER1 , P. JAGADEESAN1 , A. JAIN1 , S. K ARTHIKEYAN1 , P.K.
M ANOHARAN1 , P.K. MOHANTY1 , P.K. NAYAK1 , B.S. R AO1 , K.C. RAVINDRAN1 , H. TANAKA1 , S.C.
T ONWAR1 , K. HAYASHI2 , Y. H AYASHI2 , N. I TO2 , S. K AWAKAMI2 , T. M ATSUYAMA2 , M. M INAMINO2 ,T. N ONAKA2 , S. O GIO2 , T. O KUDA2 , A. O SHIMA2 , N. S HIMIZU2 , T. YOSHIKOSHI2 , BADRUDDIN3 , R.
H ASAN3 , S HAKEEL A HMAD3 , A.P. M ISHRA4 , P.K. SHRIVASTAVA4 , R KOUL5 , G.N. S HAH5 , T.A.
M IR5 , S. M UFTI5 , S. R AHA6 , S. G HOSH6 , P. J OARDER6 , S. S AHA6 , K. BORUAH7 , P. DATTA7 ,P.K. BORUAH7 , A.G. BARUAH7 , R. BAISHYA7 , J. S AIKIA7 , D. BANERJEE8 , P. S UBRAMANIAN8 , A.
B HADRA9 , S. K UMAR10 , R. AGARWAL10 , S.K. DUBEY10 , P. B HATTACHARJEE11
1Tata Institute of Fundamental Research, Mumbai, India
2Osaka City University, Osaka, Japan
3Aligarh Muslim University, Aligarh, India
4APS University, Rewa, India
5Bhabha Atomic Research Centre, Mumbai, India
6Bose Institute, Kolkata, India
7Gauhati University, Guwahati, India
8Indian Institute Astrophysics, Bangalore, India
9North Bengal University, Siliguri, India
10R.D. University, Jabalpur, India
11Saha Institute for Nuclear Science, Kolkata, India
gupta@grapes.tifr.res.in
Abstract: GRAPES-3 experiment employs a high-density array of scintillators and a large area tracking
muon telescope. The GRAPES collaboration is being expanded with addition of several major facilities.
These include a Cerenkov telescope and low frequency dipole array for the measurement of shower energy. Addition of several modules of muon telescopes to cover a larger area, expansion of the scintillator
array with greater separation between detectors. Installation of a neutron monitor is also planned. The
current status of the experiment would be summarized.

Introduction
The current GRAPES-3 experiment comprises atotal of 400 (each 1 m2 ) plastic scintillator detectors deployed with a separation of 8 meters, torecord the density of particles in cosmic ray showers, making it the one of the highest density EAS
arrays[1]. The GRAPES-3 experiment also hasa large 560 m2 area, tracking muon detector [2].
New results on the primary cosmic ray composition [3, 4] and on solar flares are being reported
during this conference. It is proposed to enhance
the sensitivity of this experiment by adding several new components and also by increasing thearea covered by this array. In order to achieve thisgoal an expanded GRAPES collaboration has been
formed with the participation of 9 new groups fromwithin India. We invite enhanced participation ofphysicists from other countries. We have developed new instrumentation to carry out the expansion in a cost effective manner as detailed below.
Analysis of the data from the existing GRAPES-3
experiment shows excellent sensitivity to the composition of primary cosmic rays in the knee region (1015 - 1016 eV) using muon multiplicity distribution from the large area tracking muon detector. Already the sensitivity of present experimentrequires tracking of changes in atmospheric pres-S TATUS OF GRAPES COLLABORATION

sure, at level of 0.1 mm of Hg, since it significantly affects the rate of detected cosmic ray showers. With the expanded GRAPES array it is hopedthat a consistent picture of the hadronic interaction
model and the nuclear composition of primaries
should emerge in the knee region. A significant
outcome of this exercise has been, the appearanceof possible domination of iron and other heavier
nuclei in the primary cosmic ray composition beyond 1015 eV.
GRAPES-3 has provided new information on various solar phenomena including Forbush decreases(Fds) and cosmic ray anisotropy. In particular Fd
produced by a X17 class solar flare of 28 October 2003 [5] was studied in detail due to tracking
feature of muon telescope, providing evidence fortime evolution, during onset phase of this event.
We were able to measure rigidity dependence ofmagnitude of Fd as a function of geomagnetic cutoff, from this single instrument.
A special workshop on Astroparticle physics inheld in Ooty, India in December 2006 has recommended a major expansion of GRAPES-3 by utilizing present GRAPES-3 experiment as a nucleating centre for research in Astroparticle physics inIndia by a major infusion of new funding.

Physics goals and the expansion of theGRAPES experiment
Study of nuclear composition and energy spectrum of primary cosmic rays from well below the‘knee’ from 3×1013 ) to ultra-high energy of 1018
can only done from ground based indirect observations with sophisticated air shower arrays. Measurements made with large arrays of detectors are
expected to provide more definitive information on
this subject.
It is proposed to expand GRAPES-3 shower arraywith detectors over an area of 1 km 2 in a gradedmanner using high quality plastic scintillators developed in-house. We also propose to more thandouble the area of muon detectors to 1200 m2 . It isalso proposed to add detection of radio waves andCerenkov component in showers using equipment
developed within our collaboration. Almost all ofelectronics for signal processing and data record-ing is being designed within the group. Resultingin considerable cost savings.
This expansion would permit nuclear composition
to be pinned down over an energy range from
3×1013 to 1018 eV, using this single instrument.
An additional advantage of this approach is relative freedom from the systematic errors that cropup, when the data are combined from a number ofexperiments, located at different geographic sites
on the globe. The configuration of the detectors
at larger separation consisting of multiple scintillators would permit precision measurement of time
spread of shower particles at large distances fromthe core. Thus each of these detectors would function as a mini-array and time spread is expected toprovide an additional handle on the measurementof primary energy of the particle.
For a given initial energy, energy per nucleon islower for a heavier nucleus as compared to thelighter nuclei. This results in production of larger
number of pions and their subsequent decay gives
rise to a greater flux of muons at observational
level, compared to the lighter nuclei. Thus, muoncontent of a shower is an excellent tracer of massof primary cosmic ray. Existing GRAPES-3 experiment has already demonstrated this capabilityof large area muon detectors, to measure the nuclear composition of primary cosmic rays. This is
likely to get further enhanced with the addition ofanother 500-600 m2 muon detector.
Since hadron component in a shower contains asignificant fraction of total energy, measurementof hadronic component also provides an input formodeling hadron interactions in the forward region. Combined electron, muon and hadron information would provide a sharper handle to constrain the nuclear composition. It is proposed to install a 6-8λ deep, highly segmented (50cm×50cm)
iron-scintillator calorimeter to detect and measure
the hadron component. Initially a 36 m2 areacalorimeter would be built which would be subsequently increased to 100 m2 .
Precision measurement of cosmic ray energy spectrum has the potential to detect fine spectral features in the energy spectrum. Although the ‘knee’in cosmic ray spectrum has been observed by alarge number of groups, detailed studies of finestructure in the energy region 10 15 -1016 eV arenecessary to gain insights into the mechanism re-30 TH I NTERNATIONAL C OSMIC R AY C ONFERENCE

sponsible for ‘knee’. Such studies on the size spectrum of air showers should provide valuable newinformation.
At present, primary energy is determined from a
fit to measured lateral distribution of air shower atobservational level, which suffers from significant
uncertainties due to fluctuations in development ofair shower. An excellent measurement of energy
can be made from Cerenkov radiation in shower,
as it represents a calorimetric measurement of total energy. We propose to add a Cerenkov detector to augment our array. At still higher energies≥1017 eV, electrons and positrons in a shower produce radio waves at low frequencies through Geosynchrotron emission in geo-magnetic field. We
propose to set up an array of wide-band dipoles
to detect this radio emission, which could provide
an excellent measure of primary energy. Unlike aCerenkov telescope, the dipole arrays have a muchlarger duty cycle.
We propose to also investigate solar phenomena such as Solar Flares, Coronal Mass Ejections(CMEs) and Cosmic Ray Modulation. Sun is thenearest astrophysical laboratory where almost all
known processes of acceleration of particles involving shock acceleration, magnetic reconnection
and various plasma instabilities operate, to produce
the solar wind. Solar wind is being effectively
monitored using a world-wide network of spaceand ground-based instruments. The solar windplays a crucial role in determining space weather.
During a major flares, huge transient fluxes of highenergy particles are injected into interplanetary
space including neutrons, protons and other nuclei.
Solar particles above 20 GeV, produce muons inatmosphere, which are readily detected by groundbased muon detectors. Study of solar flare particles through muons has become an important tool
because of large area of tracking muon detectors.
Lathough present studies have provided very valuable information on the physics of the solar flareswith unprecedented precision, a lot is unknown
about mechanism responsible for acceleration ofparticles during a flare. Round-the-clock high energy observations of solar flares using muons alongwith other information at lower energy is expected
to shed new light on the functioning of the solaraccelerator.

Global network of neutrons Monitors covering
a range of geomagnetic latitudes and longitudes provide information on solar phenomenon
on large scale due to their omni-directional response. In contrast muon telescopes such as
those in GRAPES-3 have a better angular resolution and are better suited for study of small scaleanisotropies in space. Thus these two instruments
provide information, which is complementary. At
present, only one Indian Neutron monitors is operating at Gulmarg. It is proposed to install neutrons monitors at Ooty to provide a better coverage.
GRAPES-3 muon telescope with an angular resolution of ∼9circ , is well suited for study of episodic
decreases (Fds) and for mapping interplanetary
ejecta in 3-D responsible for these events. A combination of neutron monitors and tracking muon
telescopes at Ooty would be a unique tool for studyof; episodic modulation, 3-D features of Fds andinterplanetary structures responsible for them, Precursor (e.g. loss cone) anisotropies, cosmic raydensity gradients in space, effects of solar windplasma and interplanetary magnetic field on highenergy cosmic rays and the physics of solar modulation.
Episodic emission of ultra-high energy (UHE >50TeV) radiation from periodic sources such as X-raybinaries and pulsars etc. is also an important component of this experiment. UHE photons are expected to be secondaries, produced in interactionof charged particles at sites of their acceleration.
High energy electrons through bremsstrahlung andfrom decay of neutral pions produced in interaction of protons produce UHE photons in compact
sources. Past observations have shown emissionof episodes of sporadic emission of UHE radiationfrom X-ray binaries and radio pulsars. The operation of a large and sensitive expanded GRAPES-3
array should permit identification of episodic emission with much greater confidence by rejection ofa large fraction of charged cosmic ray backgroundusing muon component in the shower.
Exotic phenomena such as bursts of ultra-high energy (>3×1013eV) radiation during γ-ray bursts,
or during exploding phase of primordial Hawking
black holes etc. may also be studied. Models ofrelativistic dust grains impacting Earth and producing γ-ray bursts have been proposed. Such models
predict observation of γ-ray bursts over huge dis-S TATUS OF GRAPES COLLABORATION

tances which could be detected by deploying a network of mini-GRAPES type arrays at several locations in India proposed by participating groups.
Initial proposal envisages setting up of four sucharrays at Darjeeling, Gulmarg, Guwahati besides
at Ooty. This concept was also endored at the joint
meeting of Indian funding agencies held in 2006.
Highest energy cosmic ray (CR) particles>1020 eV are expected to to produce a diffuse
flux of 100 TeV γ-rays from their interaction withcosmic microwave background (CMB) photons
resulting in ‘GZK cutoff’ in CR energy spectrumat ∼1020 eV. Experimentally it is difficult todistinguish γ- from CR induced showers withground based observations. However, several
groups in recent times have managed to placestringent upper limits on diffuse flux of γ-ray
which seem to question the validity of certain
models in the energy region from several tens ofTeV to a PeV. It is known that the muon contentof γ-ray showers, is <5% as compared to the proton/nucleus initiated showers. Therefore, showers
unaccompanied by muons (µ-poor), provide an
efficient mechanism for identification of γ-ray
showers. Proposed expansion of muon detector
would allow a separation between γ- and proton
induced showers and determination of the ratio of
γ-ray to protons (γ/p) to unprecedented precision.

Conclusions
Due to its dense detector configuration and thepresence of a very large area tracking muon detector and its high altitude, and in view its proposed
expansion the GRAPES array would offer advantages. These include enhanced ability to mesure
cosmic ray composition, its spectral shape and sensitivity to episodic phenomena. For astrophysical observations unique location of Ooty near theequator offers a view of both northern and southernskies, including the galactic centre region. The latitude difference of about 5-6 hours relative to Europe and 10-12 hours relative to Americas, makes
it possible to monitor certain UHE sources andother episodic or transient UHE emissions at a time
when they are not in field of view of other experiments elsewhere. This makes GRAPES-3 experiment a valuable component of the international

scene with great potential to contribute to the fieldof UHE astrophysics.

Acknowledgements
We thanks technical and other staff members ofGRAPES collaboration in India and Japan for theircontributions in setting up and running the experiment.

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